The present invention relates to a hybrid drive system and a drive control method.
There exists a hybrid drive system that is structured to drive a vehicle using the drive force from both the engine and the rotating electric machine (i.e., motor). The vehicle equipped with the hybrid drive system is normally driven in a fuel economy drive mode where the operation points of the engine and the rotating electric machine are determined while giving priority to fuel consumption. In the fuel economy drive mode, the engine is operated on its maximum efficiency line while giving priority to fuel consumption. That is, the optimum mileage control is executed. Therefore, it is often the case that the rotating electric machine is operated at the operating point with less efficiency, resulting in heat generation.
Japanese Unexamined Patent Application Publication No. 2006-46576 discloses a method for avoiding the heat generation of the rotating electric machine (especially the motor) by detecting the motor temperature so as to switch the drive mode to a mode that reduces the load exerted to the rotating electric machine. The method includes such drive modes as a fuel economy infinitely variable gear ratio mode (corresponding to the fuel economy drive mode), the fixed gear ratio mode, and the motor efficiency-priority infinitely variable gear ratio mode. Switching of those drive modes may be performed based on such indexes as the “heat generation temperature change” and the “heat generation temperature.”
For switching the drive mode based on the “heat generation temperature,” the heat generation temperature limit (second limit temperature MAX2) and the heat generation avoidance temperature (first limit temperature MAX1) are specified. The heat generation temperature limit stands for the “limit temperature when the heat generation temperature of the rotating electric machine is allowed to gradually increase.” The heat generation avoidance temperature stands for the “temperature lower than the second limit temperature MAX2, for example, the temperature that may reach the second limit temperature after a predetermined duration.”
The drive mode transition from a fuel consumption-priority infinite mode to a motor efficiency-priority infinite mode has been proposed in order to establish the relationship of the heat generation temperature limit (second limit temperature MAX2) that is greater than or equal to heat generation temperature that is greater than or equal to heat generation avoidance temperature (first limit temperature MAX1). The drive mode transition further to the fixed gear ratio mode is also proposed in order to establish the relationship of the heat generation temperature that is greater than heat generation temperature limit (second limit temperature MAX2).
In the aforementioned process, the heat generation avoidance temperature (first limit temperature MAX1) is preliminarily fixed to a predetermined single value.